Here's why:

* Differential Erosion: Mushroom rocks are formed when softer rock at the base of a rock formation is eroded at a faster rate than the harder rock at the top. This is due to the different rates at which different types of rock weather and erode.

* Wind and Water: The primary agents of erosion in this case are wind and water. Wind carries abrasive particles that sandblast the rock, and rain can dissolve and wear away the softer rock.

* The Shape: The "mushroom" shape is created because the softer base erodes away faster, creating a "cap" on top of a thinner stem.

Deposition would be the process of adding material, which is not how mushroom rocks are formed.